1. Field of the Invention
The present invention relates to a control device for a four-wheel drive vehicle configured to distribute the output torque of an engine to main drive wheels and to auxiliary drive wheels.
2. Description of the Background Art
As a four-wheel drive vehicle, there is known a vehicle in which a power unit installed in a vehicle body front portion is provided with a power transfer unit for driving rear wheels. The power unit includes an engine, a transmission, and a front wheel differential gear assembly. The power unit drives left and right front wheels as main drive wheels. The power transfer unit is coupled to a propeller shaft extending in the front-rear direction of the vehicle body. A rear wheel differential gear assembly is disposed at a rear end of the propeller shaft so as to transmit power to left and right rear wheels as auxiliary drive wheels, as well as to the left and right front wheels.
An electromagnetic coupling unit capable of changing a transmission torque may be disposed between a propeller shaft and a rear wheel differential gear assembly. Completely coupling the coupling unit switches the vehicle to a four-wheel drive state in which a torque is equally transmitted to front wheels and rear wheels. Completely releasing the coupling of the coupling unit switches the vehicle to a two-wheel drive state in which the driving force is transmitted only to the front wheels. Controlling the degree of coupling of the coupling unit between a completely coupled state and a completely released state makes it possible to adjust torque distribution with respect to the rear wheels depending on the degree of coupling of the coupling unit.
A power transfer unit may employ a pair of bevel gears engageable with each other in order to transmit power from a differential case of a front wheel differential gear assembly whose axis extends in the vehicle width direction to a propeller shaft whose axis extends in the front-rear direction of the vehicle body. Specifically, a bevel gear disposed on the axis of the differential case, and a bevel gear disposed on the axis of the propeller shaft which is always engaged with the bevel gear are used.
The output torque of the engine accompanies torque fluctuation, which is generated at a frequency by intermittent explosions in each of the cylinders. On the other hand, there exists a resonant frequency with respect to torsional vibration in the rotating direction of the shaft in a torque transmission mechanism including a transmission, a front wheel differential gear assembly, a power transfer unit, a propeller shaft, a coupling unit, and a rear wheel differential gear assembly. Therefore, when the frequency of torque fluctuation coincides with the resonant frequency of the torque transmission mechanism, the torsional vibration in the torque transmission mechanism may increase.
In a two-wheel drive state in which the coupling is released and the output torque is transmitted only to the front wheels, a rear wheel torque transmission mechanism that is a region from the paired bevel gears of the power transfer unit to the rear wheels is rotated in a power non-transmission state. If torsional vibration increases in this state, tooth surface separation (a state that gear engagement is released) is likely to occur discontinuously between the paired bevel gears of the power transfer unit of the rear wheel torque transmission mechanism. When the tooth surface separation occurs, abnormal noise by gear rattle is generated. This may be one of the causes of noise in the vehicle.
Meanwhile, there is proposed an idea in which the degree of coupling of a coupling unit is controlled in an operation range of an engine in which a torque transmission mechanism resonates, and a torque larger than torque fluctuation is transmitted to rear wheels by imparting a load to a rear wheel torque transmission mechanism so as to prevent rotation of the rear wheel torque transmission mechanism in a power non-transmission state. According to this configuration, even if torsional vibration increases in the torque transmission mechanism, it is possible to suppress tooth surface separation between a pair of bevel gears of a power transfer unit to thereby suppress generation of abnormal noise by gear rattle.
For instance, Japanese Unexamined Patent Publication No. 2001-277881 discloses a four-wheel drive vehicle provided with an engine, a transmission, a front wheel differential gear assembly, a power transfer unit, a propeller shaft, a coupling unit, and a rear wheel differential gear assembly, in which torque distribution between front wheels and rear wheels is changed in order to suppress generation of abnormal noise (gear rattle) by transmission of abnormal vibration of the engine to a rear wheel torque transmission mechanism when the engine is in an abnormal vibration (knocking noise) generation range.
However, the magnitude of torque fluctuation changes depending on an operation state of the engine. When the torque distribution is controlled without considering a change in torque fluctuation, fuel economy of the engine may be deteriorated due to an increase in drive loss resulting from torque distribution with respect to the rear wheels.